The use of ink-jet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today's ink-jet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving ink-jet print quality. In general, ink-jet print quality still falls short of that produced by more expensive technologies such as photography and offset or gravure printing. A surge in interest in ink-jet imaging (e.g., the rendition of pictures) has resulted in the need to produce near photographic quality printed images at a reasonable cost. The challenge remains to further improve the image quality and lightfastness of ink-jet printed images, especially images containing skin tones without increasing their cost.
Color ink-jet printers typically use three inks of differing hues: magenta, yellow, and cyan, and optionally black. The particular set of colorants, e.g., dyes, used to make the inks is called a "primary dye set." A spectrum of colors, e.g., secondary colors, can be generated using different combinations of the primary dye set. In printing processes such as lithography, dye transfer, and some types of thermal transfer, it is possible to vary spot size so that less is demanded of the primary colored inks when producing a good secondary. When the color inks are used in a binary printing device, that is, one in which a dot of color is either present or absent, e.g., a thermal ink-jet printer, the ability of the primaries to give recognizable secondaries is even more important.
If such an ink is to be used in an ink-jet printing device, characteristics such as crusting, long term stability, and materials compatibility must also be addressed. If the ink is to be used in a thermal ink-jet printer, the further constraint of being thermally stable (kogation-resistant) is added.
In general, a successful ink set for color ink-jet printing must be compatible with the ink-jet pen and printing system. Some of the required properties for the ink-jet ink include: good crusting resistance, good stability, the proper viscosity, the proper surface tension, little color-to-color bleed, rapid dry time, no negative reaction with the vehicle, high solubility of the dyes in the vehicle, consumer safety, low strike through, high color saturation, and good dot gain.
Furthermore, the color characteristics of the colorants play an important role in the quality of the printed ink-jet image. Color ink-jet inks have been designed and tailored for printing business graphics such as depiction of information in the form of "pie-charts" and "bar-charts." U.S. Pat. Nos. 5,108,504; 5,116,409; 5,118,350; 5,143,547; 5,145,519; 5,185,034; 5,198,023; 5,534,051; and 5,536,306 disclose yellow dyes, namely acid yellow 23, basic yellow 13, basic yellow 51, and direct yellow 86, for use in ink-jet inks. These dyes, although suitable for business graphics, do not provide a "true" representation of skin tones.
Any given perceived color can be described using any one of the color spaces, such as CIELAB, and Munsell, as is well known in the art. For example, in the Munsell color space a given color is defined using three terms, Hue, Value, and Chroma. Similarly, as illustrated in FIGS. 1 and 2, in the CIELAB color space, a color is defined using three terms L*, a*, and b*. L* defines the lightness of a color, and it ranges from zero (black) to 100 (white). The terms a* and b*, together, define the hue. The term a* ranges from a negative number (green) to a positive number (red). The term b* ranges from a negative number (blue) to a positive number (yellow). Additional terms such as h.degree. (hue angle) and C* (chroma) are used to further describe a given color, wherein EQU h.degree.=tan.sup.-1 b*/a* equation 1 EQU C*=(a*.sup.2 +b*.sup.2).sup.1/2 equation 2
One attribute which is particularly important in imaging (i.e., color rendition of pictures) is "color trueness." Color trueness describes how close the ink-jet printed image is to the original or primary object, such as the skin tone of a human being or the color of the sky. In addition, it is important that the printed image preserve its color characteristics over time as it is exposed to light. Thus, there is a need for more lightfast colorants. It is also of particular importance that when the image does fade, the fading occurs uniformly across all printed colors, thus minimizing the hue shift between the more faded and the less faded colors.
In the CIELAB color space, delta E (.DELTA.E) defines the difference between two sets of colors, such as the color of the printed image and the original object, or the color of the original printed image and the color of the same image after lightfade--the higher the .DELTA.E number, the more difference between the two colors EQU .DELTA.E=(.DELTA.L*.sup.2 +.DELTA.a*.sup.2 +.DELTA.b*.sup.2).sup.1/2equation 3
Inks are known which possess one or more of the foregoing properties. However, few inks are known that possess all the foregoing properties, since an improvement in one property often results in the degradation of another property. Thus, many inks used commercially represent a compromise in an attempt to achieve an ink evidencing at least an adequate response in each of the foregoing considerations. Accordingly, investigations continue into developing ink formulations which have improved properties and which do not improve one property at the expense of the others. Thus, challenge remains to further improve the image quality and light fastness of the ink-jet prints without sacrificing pen performance and reliability.